setup.sh: use -exec rather than -execdir
setup.sh: use -exec rather than -execdir

Busybox's `find` does not support `-execdir`, so let's uses `-exec`
instead, in the spirit of requiring the minimum features needed for
the task.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs